With the development of efficient light emitting diodes (LEDs) that emit blue or ultraviolet (UV) light, it has become feasible to produce LEDs that generate white light through phosphor conversion (pc) of a portion of the primary emission of the LEDs to longer wavelengths. These white light sources are so-called phosphor converted LEDs (pcLEDs). Conversion of primary emission of the LED to secondary emission with longer wavelengths is commonly referred to as down-conversion of the primary emission. “Primary light” or “primary emission” refers to light emitted by a light emitting diode (LED) and “secondary light” or “secondary emission” refers to light emitted by a phosphor, the so-called conversion element. In the following, the ratio between the non-converted (transmitted) primary light and the secondary light is denoted as light conversion power. The non-converted primary light combines with the secondary light to produce white light. The color point of the white light is determined by the light conversion power of the conversion element and the wavelength of the primary light. “Light conversion power” refers to the number of emitted photons of the light conversion element compared to the number of transmitted photons of a primary light. However, due to commonly present process variations the primary and secondary power ratios slightly deviate between individual pcLEDs resulting in visible deviations of color point. While the efficacy improved at lot in the last decade, the color consistency remains as one of the last obstacles for white LEDs to penetrate the general illumination market.
Before mounting the conversion elements current pcLED manufactures classify the LEDs (so-called binning) according to the characteristics of the emitted primary light, e.g. according to the peak wavelength (wavelength of maximum intensity). However, after mounting of conversion elements comprising the same conversion material on top of the LEDs taken from one class (BIN), the resulting pcLEDs show a large variation of their color points. This effect is caused by a varying conversion power between individual conversion elements. Beside the composition of a conversion material, the conversion power depends on additional parameters such as material density, scattering behavior and geometrical properties. Current process tolerances during production of conversion elements comprising the same conversion material lead to visible variations of the conversion power between individual conversion elements. Therefore also the conversion element needs to be classified before mounting it to a LED (so-called binning).
EP 1605526 discloses a pcLED, where the conversion element is produced in a sheet that is separated into individual conversion elements, which are bonded to LEDs to form pcLEDs. The conversion elements are selectively matched with a semiconductor LED to produce a desired mixture of primary and secondary light resulting in a desired color point of the pcLED. To determine the conversion properties of each conversion element, a sheet consisting of several conversion elements is illuminated with primary light and the mixture of converted and primary light is measured. Measurements performed on sheets of conversion elements are advantageous with respect to a short measuring time but the measurement accuracy is not sufficient.
Measurements performed on single conversion elements avoid this problem, but are time-consuming and therefore not preferred. The required time to measure a certain number of conversion elements increases significantly and prevents an application in mass production. Additionally, the construction of the sample holder may influence the measuring result by shading and/or reflecting effects resulting in an inaccurate primary to secondary light intensity ratio.
Matching of conversion elements with inaccurate determined conversion power to semiconductor LEDs leads to varying color points between different pcLEDs. To penetrate the general illumination market, a better color consistency of pcLEDs is required.